The present invention relates to polyurethane foams. More particularly, the present invention relates to polyurethane foams having greatly enhanced absorbent capabilities, i.e., superabsorbent polyurethane foams, which are based on quasi-interpenetrating polymer networks.
Polyurethane foams are, of course, well known to those having ordinary skill in the art. Indeed, a voluminous body of literature has accumulated over the years as researchers explored combinations of starting materials and correlated starting materials with foam properties. Moreover, numerous efforts have been made to either modify the physical properties of polyurethane foams or to prepare specialized polyurethane foams having unique properties. A particularly sought-after property is increased water absorbency. Polymers having this property often are referred to as hydrogels or superabsorbents.
The nature and utility of superabsorbent are illustrated by U.S. Pat. No. 4,449,977, although it will be appreciated by those having ordinary skill in the art that numerous other references also could be cited. According to this reference, an apparently desirable feature of a superabsorbent is the presence of acrylate or methacrylate groups which can be salts, amides, esters, or the free acids.
As a practical matter, many hydrogels are based on acrylate and methacrylate polymers and copolymers. See, by way of example only, U.S. Pat. Nos. 2,976,576, 3,220,960, 3,993,616, 4,154,898, 4,167,464, 4,192,727, 4,192,827, and 4,529,739. The last-cited patent is of particular interest since the disclosed water-absorbent polymers are foamed. Other hydrogels are based on starch or a modified starch, as shown by U.S. Pat. Nos. 4,069,177, 4,076,663, 4,115,332, and 4,117,222. Still other hydrogels are based on poly(oxyalkylene) glycols; see, e.g., U.S. Pat. No. 3,783,872. Hydrogels prepared from hydrolyzed crosslinked polyacrylamides and crosslinked sulfonated polystyrenes are described in U.S. Pat. No. 4,235,237. Finally, polymers based on maleic anhydride (or similar compounds) have been described in U.S. Pat. Nos. 2,988,539, 3,393,168, 3,514,419, 3,557,067, and 4,401,793. U.S. Pat. No. 3,900,378 is of interest since it describes hydrogels from radiation crosslinked blends of hydrophilic polymers and fillers, many of the polymers being those described in the foregoing patents. However, such materials are not necessarily well suited for the uses described in U.S. Pat. No. 4,449,977, supra.
Polyurethane hydrogels are, of course, known and frequently are based on the reaction of a poly(oxyalkylene) polyol with a diisocyanate. However, relatively few of such hydrogels contain acrylate or methacrylate moieties, or even carboxylate groups. Examples of carboxylate-containing polyurethanes and polyurethane hydrogels are noted below.
U.S. Pat. No. 3,928,299 describes the reaction of a hydroxy group-containing polymer with an unsaturated isocyanate. Suitable hydroxy group-containing polymers can be obtained by the polymerization of hydroxyalkyl esters of acrylic acid or alpha-alkyl-substituted acrylic acids or by the copolymerization of these compounds with other vinyl or vinylidene compounds. Suitable hydroxyalkyl esters are the monoesters of the foregoing acids with ethylene glycol, propylene glycol, propane-1,3-diol, butanediol, diethylene glycol, and higher polyethylene glycols. Such polymers also can be polyesters of polybasic aliphatic or aromatic carboxylic acids with polyhydric alcohols, polyurethanes which contain hydroxy groups, or epoxy resins which contain hydroxy groups. The resulting polymers are crosslinkable by vinyl polymerization and are useful for the preparation of coating and molded products. Foams, especially polyurethane foams, are not mentioned. See also U.S. Pat. No. 4,210,713.
Disclosures similar to that of the above patent are found in U.S. Pat. Nos. 3,871,908, 3,856,830, 3,954,714, and 4,082,710.
U.S. Pat. No. 4,131,602 describes radiation-curable acrylated polyurethane coating compositions. The compositions are prepared by reacting an isocyanate-terminated urethane intermediate with an amount of a hydroxyalkyl, hydroxycycloalkyl, or hydroxyaryl ester of acrylic acid or methacrylic acid which is sufficient to react with at least 80 percent of the isocyanate groups of the urethane intermediate. The urethane intermediate is the reaction product of an organic diisocyanate, an organic triol or tetraol which is either a polyester or a polyether, and an organic diol which is either a polyester or a polyether. The diisocyanate is used in an equivalent excess to the other two components. In addition, when the triol or tetraol is a polyester, the diol must be a polyether, and when the triol or tetraol is a polyether, the diol must be a polyester.
U.S. Pat. No. 4,153,778 describes acrylyl-capped urethane oligomers which readily cure by thermal or radiation means and are useful as coatings, binders, and adhesives. The oligomers are the reaction products of a poly(oxytetramethylene) diol or a polycaprolactone polyol, an organic diisocyanate, a di- or trimethylol carboxylic acid, and an acrylyl compound.
Polyester urethane-containing molding compositions are described in U.S. Pat. No. 4,287,116. Briefly, an ethylenically unsaturated monomer solution having dissolved therein a polyester urethane resin and an organic polyol polyurethane precursor is gelled by the addition of a polyisocyanate polyurethane precursor. The gelled mixture then is molded and cured by copolymerization of the ethylenically unsaturated monomer and the resin. The resin typically is a condensation product of a dihydroxy-terminated poly(oxyalkylene) bisphenol A maleate or fumarate and a polyisocyanate which is further reacted with a hydroxy-terminated ester of acrylic or methacrylic acid. The polyol precursor is a saturated aliphatic polyol or alkoxylated derivative thereof. The ethylenically unsaturated monomer can be, by way of illustration, styrene, vinyltoluene, divinylbenzene, esters of acrylic or methacrylic acid, vinyl acetate, divinyl ether, and the like. Finally, the polyisocyanate precursor is an aliphatic, cycloaliphatic, or aromatic compound having at least two isocyanate groups.
U.S. Pat. No. 4,480,079 discloses molded plastic products which are produced by the in-mold copolymerization of methyl methacrylate with a polyurethane acrylate or methacrylate. The latter material is derived from a hydroxylakyl acrylate or methacrylate by reaction of the hydroxy groups thereof with the isocyanate groups of either a polyisocyanate free of urethane groups and having an isocyanate functionality greater than two or a urethane polyisocyanate having an isocyanate functionality greater than two and derived from a polyisocyanate by reaction thereof with the hydroxy groups of an aliphatic alcohol having up to three hydroxy groups.
A series of ten patents relates to polyurethane polyene or diacrylate polymers. These ten patents, discussed briefly below, appear to relate to interpenetrating polymer networks, although such terminology does not appear to have been applied to the polymer compositions described by these references. For general discussions of interpenetrating polymer networks, see, by way of example only, D. Klempner et al., J. Elastoplastics, 5, 196 (1973); A. A. Donatelli et al., Macromolecules, 9, 671 and 676 (1976); L. H. Sperling et al., Macromolecules, 9, 743 (1976); L. H. Sperling, J. Polymer Science, 12, 141 (1977); and D. L. Siegfried et al., J. Polymer Science, 16, 583 (1978).
U.S. Pat. No. 4,359,558 discloses hydrophilic polyurethane diacrylate compositions. The compositions, which form hydrogels upon immersion in water, are prepared by reacting a diacrylate in the presence of a hydrophilic polyurethane. A free radical initiator may be present. The polyurethane typically is the reaction product of one or more diols having a number average molecular weight in the range of from about 200 to about 20,000, selected from the group consisting of ethylene glycol and long chain poly(oxyalkylene) diols, with a urethane precursor selected from the group consisting of organic polyisocyanates and nitrile carbonates in the presence of an organic tin catalyst. Optionally, polyfunctional lactone also may be present in amounts of from 0.1 to 30 percent by weight, based on the weight of the total reaction mixture. The long chain diols typically are the condensation products of either ethylene oxide or propylene oxide. The diacrylate may be prepared by reacting acrylic acid chloride (propenoyl chloride) or methacrylic acid chloride (2-methylpropenoyl chloride) with a glycol such as ethylene glycol or a condensation product of either ethylene oxide or propylene oxide. The two components are dissolved in a suitable solvent, cast as a film, and cured by heat or ultraviolet radiation. See also U.S. Pat. Nos. 4,408,023, 4,424,305, 4,439,583, 4,439,584, and 4,439,585.
Hydrophilic polyurethane acrylate compositions are disclosed in U.K. Patent Application No. GB 2,150,938A. The disclosure is similar to that of U.S. Pat. Nos. 4,359,558 et al., except that the diacrylate is replaced with an acrylate which is the monoacrylic or monomethacrylic ester of an alcohol having less than 13 carbon atoms. The preferred acrylates are stated to be hydroxyethyl acrylate, methyl methacrylate, and methyl acrylate. The polyurethane and acrylate components are dissolved in a solvent, optionally in the presence of a free radical initiator, cast as a film, and cured thermally or by ultraviolet radiation. Shaped articles can be made by removing the solvent under reduced pressure, molding the residual mixture, and curing the molded article thermally.
Hydrophilic polyurethane polyene compositions are disclosed in U.S. Pat. No. 4,454,309. The compositions are prepared by reacting a polyene in the presence of a hydrophilic polyurethane. The polyene is either a polyallyl ester of a polybasic acid or a polyacrylic or polymerthacrylic ester of a polyhydric alcohol. The polyurethane is that described in U.S. Pat. No. 4,359,558. As with the compositions of such earlier patent, the components are dissolved in a suitable solvent, cast as a film, and cured with heat or ultraviolet radiation. Alternatively, the solvent may be removed under reduced pressure and the residual mixture molded and cured thermally. See also U.S. Pat. Nos. 4,490,423 and 4,496,535.
In each of the foregoing reference, the unsaturated monomers are polymerized in the presence of an existing polymer, i.e., a polyurethane. A similar approach is disclosed in U.S. Pat. No. 4,551,486. According to the patent, hardenable dental compositions are prepared by polymerizing crosslinking oligomers in the presence of a crosslinked polymer and one or more of a filler, an initiator, and a monofunctional monomer. The crosslinked polymer can be a polyurethane, although the preferred polymers are derived from aliphatic, cycloaliphatic, phenyl, and substituted phenyl esters of acrylic acid and homologs thereof. The crosslinking agents which are useful in the preparation of the crosslinked polymer can be selected from a wide variety of polyfunctional materials. The preferred functionality apparently is an ethylenic function, presumably because the preferred polymers are prepared by the addition polymerization of unsaturated monomers. The crosslinking aligomers also tend to be polyunsaturated compounds, such as acrylic and lower alkyl acrylic acid diesters, acrylic and lower acrylic acid esters of alcohols having a second reactive function, urethane diacrylates and dimethacrylates, polyvinyl compounds, divinyl aromatic compounds, and the like. Preferred compounds include allyl acrylate, allyl methacrylate, vinyl acrylate, vinyl methacrylate, dimethallyl fumarate, N-allylacrylamide, crotyl acrylate, allyl crotonate, allyl cinnamate, diallyl maleate, acrylate and methacrylate esters of polyols, and the like.
The reverse approach is described in European Patent Application No. 85105252.2, published as No. 0,163,150. In general, a polyurethane foam is prepared in the presence of a polyelectrolyte polymer. Preferably, the polyurethane is prepared from an isocyanate-terminated poly (oxyalkylene) polyol, such as the HYPOL(.RTM.) precursors sold by W. R. Grace & Co. The crosslinking agents are selected to react with the carboxylic acid groups of the polyelectroyte polymer and include polyhaloalkanols, haloepoxyalkenes, polyglycidyl ethers, defined di- and triaziridines, and the like. The polyelectrolyte polymers tend to be polymers or copolymers of acrylic and methacrylic acid with such monomers as acrylates, methacrylates, acrylamide, olefins, vinyl aromatic compounds, styrenesulfonic acid, vinyl ethers, vinyl acetate, vinyl alcohol, maleic acid, fumaric acid, and the like. The examples used a polyacrylic acid which had been treated with sodium hydroxide.
It perhaps should be noted at this point that interpenetrating polymer networks ideally do not include any grafting of the first polymer to the second, although, as noted by Donatelli et al., some grafting may take place accidentally. Because of the selection of polymer types described in the foregoing interpenetrating polymer network reference, little, if any, grafting should have taken place.
Acrylic monomers containing carbamate (urethane) functionality are described in U.S. Pat. Nos. 3,297,745, 3,425,988, 4,129,667, and 4,279,833. An acrylic monomer containing isocyanate functionality, isocyanatoethyl methacrylate, is described in Adhesives Age, Oct. 1984. The article summarizes there areas in which isocyanatoethyl methacrylate has been used: (1) applications where the methacrylate group is polymerized first, leaving the isocyanate group for a later reaction, (2) applications where the isocyanate group is reacted with a polyfunctional material first, leaving the vinyl group for a later reaction, and (3) applications where the isocyanate group is reacted with a monofunctional reagent first to make a new monomer which can be polymerized later.
Because a significant amount of the voluminous polyurethane foam literature relates to the use as a starting material of what may be termed polyether polyols, polyglycolethers, or poly(oxyalkylene) polyols and such starting material has acquired a singularly important status in the polyurethane art, a discussion of representative reference relating thereto is deemed necessary for the sake of completeness.
One of the earliest reference describing such materials is U.S. Pat. No. 2,948,691. According to this patent, polyglycolethers having a molecular weight of at least 500 and at least two terminal hydroxy groups can be reacted with mono- or polyfunctional isocyanates to give products which may be used for producing plasticizers, lubricants, plastics, spongy materials, gel formers, thickening agents, and the like. The patent describes the preparation of hydrogels and foams.
Subsequent studies with these poly (oxyalkylene) polyols demonstrated a high suitability for the preparation of hydrogels and foams having particular properties, as illustrated by the reference described below.
U.S. Pat. No. 3,861,993 describes a composite foam scouring pad, one component of which is a hydrophilic foam composition prepared by reacting an isocyanate-capped poly(oxyethylene) polyol having an isocyanate functionality of at least two with an aqueous solution containing a blowing agent such as a polyisocyanate, a nonionic surfactant, and, when the isocyanate-capped poly(oxyethylene) polyol isocyanate functionality is about two, a crosslinking agent. The ratio of moles of water to moles of isocyanate functionality in the polyol can range from about 6.5 to about 390. The same hydrophilic foam is employed to prepare a laminated fabric as described in U.S. Pat. No. 3,874,964 and a horticultural foam structure as described in U.S. Pat. No. 3,889,417. The reticulated crosslinked polyurethane foam described in U.S. Pat. No. 3,890,254 appears to differ from that described above in that particular types of surfactants are required and the isocyanate-capped poly(oxyethylene) polyol is derived from a poly(oxyethylene) polyol having a weight average molecular weight of from about 200 to about 20,000 and a hydroxy group functionality of from about 2 to about 8. See also U.S. Pat. No. 4,160,076.
Compressed foams which are restored to their original volume in the presence of water or heat are disclosed in U.S. Pat. No. 3,903,232; see also U.S. Pat. No. 3,854,535. The foams are similar to those described in U.S. Pat. No. 3,861,993. Briefly, a mixture of from 0 to about 97 percent by weight of an isocyanate-capped hydrophilic poly(oxyethylene) polyol having an isocyanate functionality of two and an isocyanate-capped poly(oxyethylene) polyol having an isocyanate functionality of from about 3 to about 8 and a weight average molecular weight of from about 200 to about 1,500 (20,000 according to claim 1) is reacted with water, optionally in the presence of a crosslinking agent. The ratio of moles of water to moles of isocyanate groups can range from about 6.5 to about 390. See also U.S. Pat. Nos. 4,156,592 and 4,292,412 which disclose the use of such foams in the preparation of expandable fabric softener-containing articles and hydrophilic fabric softener foam compositions, respectively. Similar foams are disclosed in U.S. Pat. Nos. 4,110,508 and 4,137,200 in which the poly(oxyethylene) polyol moiety of the isocyanate-capped polyol has a weight average molecular weight of from about 200 to about 20,000. See also U.S. Pat. Nos. 4,201,846, 4,258,137, and 4,309,509 which describe the incorporation into the foam of U.S. Pat. No. 4,137,200 hydrophilic fibers prepared from vinyl alcohol homopolymers and copolymers, an epoxy resin, and an odorant, respectively. In addition, U.S. Pat. No. 4,127,516 describes the inclusion of a polyurea in the reaction mixture which yields the foams of U.S. Pat. No. 4,110,508. The polyurea is prepared by, for example, the reaction between a linear poly(oxyethylene) polyol which has been capped with a polyisocyanate and a polyamine in an organic solvent.
U.S. Pat. No. 3,904,557 describes a method for producing a multicolored polyurethane sponge. A poly(oxyethylene) polyol having a weight average molecular weight of from about 200 to about 20,000 and a hydroxy functionality of from about 2 to about 8 is capped with a polyisocyanate. At least two distinctly different coloring agents are added to at least two different portions of isocyanate-capped polyol or water. The colored portions then are reacted with separate portions of water or isocyanate-capped polyol, respectively, to form separate colored foaming masses which then are mixed together under laminar flow conditions to yield a multicolored variegated polyurethane foam.
A polyurethane hydrogel is described in U.S. Pat. No. 4,118,354. The hydrogel is produced by dispersing into an aqueous liquid phase a product obtained by the reaction of a polyisocyanate having at least two isocyanate groups with a polyether. The polyether results from the polycondensation of at least two alkylene oxides with a polyalcohol having at least two hydroxy groups and has an average molecular weight per hydroxy group of from 1,000 to 4,000. Preferably, 75 to 85 percent of the alkylene oxides is ethylene oxide. The resulting hydrogel is stated to have a greater water content and to be highly elastic and highly stable, even in the presence of a corrosive electrolyte solution.
Urethane foams having low resiliency are described in U.S. Pat. No. 4,158,087. The foams are obtained by reacting a poly(oxyalklene) urethane prepolymer containing at least 40 mole percent of oxyethylene units in the oxyalkylene portion of the prepolymer, water, and from about 40 to about 150 parts by weight on a solids basis per 100 parts by weight of the prepolymer of a synthetic polymer latex. The prepolymer is an isocyanate-capped poly(oxyethylene) polyol of the type described in U.S. Pat. Nos. 3,903,232 et seq.
U.S. Pat. No. 4,181,770 describes the preparation of a hydrophilic foam from an isocyanate-terminated branched polyethylene polyol, an isocyanate-terminated polyester prepolymer, a minor amount of a 4,4-diphenylmethanediisocyanate/polycarbodiimide liquid condensation product which has 30 percent free isocyanate groups, and water. The foam is stated to have improved firmness and scuff resistance properties.
Isocyanate-capped urethane-containing prepolymers prepared from polyols obtained from an epihalohydrin are described in U.S. Pat. Nos. 4,273,913 and 4,297,482. The polyol can be, for example, a polyalkylene glycol composed of the same or different oxyalkylene units or a mixture of different polyalkylene glycols.
U.S. Pat. Nos. 4,314,034, 4,365,025, 4,377,645, 4,384,050, and 4,384,051 describe variations of a general concept which involves mixing a resin phase and an aqueous phase. The resin phase comprises an isocyanate-capped poly(oxyalkylene) glycol of the type described in U.S. Pat. Nos. 3,903,232 et seq. and diphenylmethane diisocyanate and/or polymeric forms or isocyanate-containing derivatives thereof.
Finally, mention should be made of U.S. Pat. Nos. 3,412,054 and 4,156,066. The first patent describes water-dilutable polyurethanes which are useful as surface coatings and printing inks. Such polyurethanes contain carboxylic acid groups which can be neutralized with ammonia or an amine. The carboxylic acid groups are provided by incorporating into the polyurethane a 2,2-bis(hydroxymethyl)-substituted carboxylic acid. Examples of suitable acids include 2,2-bis(hydroxymethyl)acetic acid, 2,2,2-tris(hydroxymethyl) acetic acid, 2,2-bis(hydroxymethyl)propionic acid, 2,2-bis(hydroxymethyl)butyric acid, 2,2-bis(hydroxymethyl)pentanoic acid, and the like. The polyurethanes are prepared by known techniques, such as by adding organic diisocyanate to a mixture of a suitable carboxylic acid and a polyol polyurethane precursor.
The second patent, U.S. Pat. No. 4,156,066, discloses polyurethanes characterized by lactone groups and hydroxy groups in the polymer backbone. The polyurethanes are prepared by reacting an organic polyisocyanate with a poly(oxyalkylene) polyol and a polyfunctional lactone having excess hydroxy groups. The free hydroxy groups which are present in the formed polyurethane are available for crosslinking the polymer. The lactone groups can be hydrolyzed to form free carboxylic acid groups or carboxylate groups.
In addition to efforts directed at altering the nature of the polyurethane per se, as noted at length above, other efforts have been directed at either incorporating into the polyurethane foam a material which will give the desired property or preparing a foam of an entirely different type. Both of these approaches are illustrated by the references which are summarized in the paragraphs which follow.
U.S. Pat. No. 3,900,030 describes a polyurethane foam of approximately the same type as those disclosed in U.S. Pat. No. 3,903,232 et. seq. which has dispersed throughout the foam a particulate, water-swellable polymer containing a plurality of hydrophilic groups such as carboxy, carboxamide, sulfonate salt, or hydroxy groups. The particulate polymer is included to increase the water absorbency of the foam.
According to U.S. Pat. No. 4,377,160, a sheet or strip of a polyurethane foam is dipped first into a polyvinyl alcohol solution and then into a reactive gelling agent solution in order to gel the polyvinyl alcohol in the foam. The resulting gel-impregnated foam is useful as a cooling compression bandage.
Finally, U.S. Pat. No. 4,098,728 discloses foams prepared by the copolymerization of polyvinyl alcohol and formaldehyde. The foams are stated to be useful as surgical sponges. The patent notes deficiencies with polyurethane sponges which are related to the generally hydrophobic nature of polyurethanes. The patent also states that fast wicking and high liquid holding capacity are desirable qualities of surgical sponges, qualities which apparently are lacking in polyurethane sponges.
It is evident that the polyurethane foams of the prior art, while certainly admirable for many applications, suffer from various disadvantages for a number of uses. Many of these disadvantages are associated with the generally hydrophobic nature of polyurethanes. Consequently, in spite of the prior art efforts to prepare superabsorbent foams, there still is a need for improvements with regard to such materials.